Signal controlled switching circuits in telephone systems



2,212,960 SIGNAIJ CONTROLLED swITcHING CIRCUITS IN TELEPHON SYSTEMS m wv N AQ L.. \\\Y, Aw 1 0 I p l Nmqlmw im I -I V.

Aug. 27, 1940. sci-[OTT Filed July 12, 1939 ril' VVE/V705 By' L. SCHOTT@6M-X Patented Aug. 27, 1940 UN'E'EB STATES PATENT ortica SIGNALCONTROLLED SWITCHING CIR- CUITS IN TELEPHONE SYSTEMS Application July12, 1939, Serial N0. 283,926

l0 Claims.

The invention relates to signal wave transmission systems andparticularly to signal control switching circuits employed forcontrolling trans.- mission in telephone systems.

The invention will be specically described as applied to thevoice-operated switching circuits, commonly called Vodas (voice-operateddevice anti-singing) circuits, employed for preventing singing andsuppressing echoes in two-way telephone systems, but is adapted as wellfor use with other types of voice-operated switching circuits. As iswell known, such systems are subject to variable interfering waves ofdifferent types as regards duration and frequency of occurrence, such asroom or line noise and static, having a frequency spectrum similar tothat of the voice signals but which are of a steady nature compared tosuch signals. This noise is often of such amplitude as to tend toproduce false operation of the voice-operated switching circuits at thesensitivity required for their proper operation on voice signals.

The Vodas switching circuits at the terminals of a radio terminal systemare intended to be operated properly only by the speech waves which passthrough the terminal. To that end, the sensitivities of these circuitsmust be so adjusted as to make their proper actions possible. Thereceiving Vodas circuits are usually adjusted as sensitive as possiblefor the amount of noise or static present in the receiving signal path,whereas the transmitting Vodas circuits have their sensitivitiesadjusted in relation to the level of the speech current in thetransmitting signal path. In the case of the former circuit, thesensitivity may, but not necessarily so, remain iixed for appreciableperiods of time with predictable adjustments, whereas in the case of thelatter circuit the sensitivity may vary widely for successive talkersover about a 40-decibel range.

Particularly, when an automatic device like the Vogad (volume-operatedgain adjusting device) is used in the transmitting path to provide. inaddition to its usual function of delivering substantially constantsignal volume to the radio transmitter, that oi adjusting thesensitivity of the transmitting Vodas circuit to compensate for thevarying signal input with strong and weak talkers, diiiiculties may beencountered from abnormal noise operations of the transmitting Vodascircuit, which may impose severe penalties on -circuit performance.

It is an object of this invention to eliminate to a large extent thenoise operation difficulties of voice-operated switching circuits insuch systems,

observed as detrimental under service conditions.

A more specific object is to protect the switching relays of a relaytype, voice-operated switching circuit employed for preventing singingand suppressing echoes in a two-Way telephone system, against excessivefalse operation by noise of the diierent types encountered in such asystem.

These objects are attained in accordance with the inventionA by acircuit arrangement comprisinga plurality of control circuits controlledfromv diiTerent parts of the voice-operated switching circuit, or fromthat circuit and the Vogad in combination, having such delays inoperation as to be respectively responsive to abnormal false operationof the voice-operated switching circuit by noise of one of the differenttypes, to adjust the sensitivity of the switching circuit sufficientlyto cause its release.

The invention will be better understood from the following detaileddescription thereof when read in conjunction with. the accompanyingdrawing, the single iigure of which shows 'schematically, and in partdiagrammatically, one terminal of a radio telephone system embodying oneform of the invention.

First, the diierent types of noise difficulties whichl experience hasindicated are most likely to be encountered under service conditions atthe terminal of a radio telephone system will .be described briefly.These include:

(1) Roomnoise, the most troublesome cases of which are generally causedby the traiiic operator leavingher talking set connected to the terminalduring idle circuit periods. This noise, generallyv below the normaltalkingy level, may occasionally become high enough to operate the gainincreaser ofthe Vogad and cause the gain in that device to rise, inVturn causing the vnoise'at the transmitting Vodas input to become highenough to operate the transmitting switching relays. In many cases, thisnoise has caused solid o-peration of the relays which thus blocks oficompletely any received transmission until the transmitting relays arereleased.

(2) Intermittent noises, whichmay or may not affect the Vogad but whosefrequency of occurrence is such as to effectively. bridge the normalreleases of the transmitting Vodas'relays, particularly the nal relay inthe chain operating to disable the receiving voice transmission path,which is the slowest to release, causing excessive operation times forthese relays.

(3) Steady noise of a type suchvas would hold 55 the transmitting masterrelay operated for an excessive period.

The Vodas and the Vogad operate `on these types of noise in acharacteristic manner generally quite distinguishable from the manner inwhich they operate on pure speech current. The circuits of the inventionto be described in connection with the drawing make use of thecharacteristic differences in operation to effect automatic release fromthe noise troubles when they occur.

The drawing shows schematically one terminal of a two-way radiotelephone system comprising a four-wire circuit connecting a telephoneline to a radio transmitter and a radio receiver which may be of anywell-known type or construction. The four-wire circuit comprises atransmitting circuit TC leading to the radio transmitter i and areceiving circuit RC leading from the radio receiver 2. The input oi thetransmitting circuit TC and the output of the receiving circuit RC areconnected in wellknown manner by the hybrid coil H1 and associatedbalancing network N1 in substantially conjugate relation with each otherand in energy transmitting relation with the two-wire line section 3which may be connected with a two-way telephone line.

The transmitting circuit TC includes, in order, connected between thehybrid coil H1 and the radio transmitter l, a vario-repeater 5 which,with the associated control circuits to be described later, constitutesan automatic volume control device known as a Vogad, a delay circuit 6and one or more ampliers. The receiving circuit RC includes, in order,connected between the radio receiver 2 and the hybrid coil H1 thevariable loss pad l, a delay circuit` and a volume control device 9. Thetransmitting circuit TC is normally disabled at a point between theradio transmitter l and the delay circuit 5 by a short-circuitingconnection Ii), whereas the receiving circuit RC is normally operativedue to a normal low loss condition of the suppressor loss pad 7.

Connected across the circuit TC at a point between the vario-repeater 5of the Vogad and the delay circuit 6, is the input of the transmittingVodas circuit TV comprising an adjustable resistance loss pad li forobtaining desired initial adjustment of the sensitivity of thetransmitting Vodas, a wave-operated switching device i2 which may be avoltage-operated amplier-detector circuit such as is well known in theart, and a plurality of mechanical switching relays including the masterVodas relay i3, connected to the output of the control device l2, so asto be simultaneously operatively energized in response to operation ofthe latter device by outgoing speech signals in the transmitting circuitTC, providing their energizing circuit has not been previously disabledby the receiving Vodas circuit RV in response to prior signals in thereceiving circuit RC received from the distant radio terminal in themanner described below. The master Vodas relay I3, when operated, closesan energizing circuit for relay 3'! and the switching relays lli and I5so that these relays are operated. Relay lil, when operated, opens itsnormally closed contacts to break the short-circuiting connection l()across the circuit TC so as to render that circuit operative to transmitthe speech signals which meanwhile have been delayed in delay circuit 5.Relay l5 when operated opens its normally closed contacts to put theloss pad 7 in the receiving circuit RC in a high loss condition, so aseiectively to disable that circuit to prevent singing and suppressechoes.

Relays M, 3l and i5 have progressively lower bias winding effects forthe purpose of their respective slow release actions which are otherwisecontrolled by the charging of condenser 53. After relay i5 releases,relay lll releases in about .l2 second, relay 3l in .15 second and relayi5 in .17 second. Relay 3l! releases and quickly stops the charging ofcondenser 53, thus insuring that the slower release of relay i5 is soonthereafter eiected.

The suppressor loss pad l may be of any type adapted to be changed froma low loss condition to a high loss condition by operation of a relay.The particular loss pad illustrated operating in this manner is similarto the one disclosed and claimed in the Silent Patent 1,749,851 issuedMarch 11, 1930.

The receiving Vodas circuit RV, the input of which is connected acrossthe receiving circuit RC at a point between the loss pad 'l and thedelay circuit 8, comprises the adjustable resistance loss pad it forobtaining a desired initial sensitivity adjustment of the receivingVodascircuit, the wave-operated control device il' which may be avoltage-operated amplier-detector circuit like the transmitting devicei2, and a chain of switching relays including a master relay i8 which isoperated in response to operation of the control device Il by appliedvoice waves, and a relay i5 operatively energized by operation of themaster relay i3 and operating to break the energizing circuit for therelays including the master Vodas relay i3, controlled from the outputof the transmitting control device i2 in the transmitting Vodas circuitTV.

The Vogad in the transmitting circuit TC may be of any oi the well-knowntypes. The particular Vogad illustrated is essentially the same asdisclosed in Mitchell et al. Patent 2,019,577 issued November 5, 1935.It includes the vario-repeater 5 comprising two three-electrodeamplifying vacuum tubes i connected in push-pull in the transmittingcircuit TC, having a gain regulating condenser Zii common to the controlgrid-cathode circuits of the two amplifying tubes, and asssociatedcontrol circuits comprising the gain decreaser circuit 2i, the gainincreaser circuit 22 and the gain increase disabler circuit 23.

The gain decreaser circuit 2i includes the three-electrode vacuum tubedetector 2li having its input connected across the transmitting cir-'cuit TC beyond the output of vario-repeater 5, y

and its output connected across the gain-regulating condenser 2u. Thiscircuit is arranged to operate in response to an increase in the levelof the waves in the output of vario-repeater 5 above a certain desiredvalue to reduce the charge on the condenser 25, so as to cause the gainof the vario-repeater 5 to be decreased in proportion to the increase inthe output level thereof over the desired value.

The gain increaser circuit 22, the input of which is coupled to thetransmitting circuit TC at a point in front of vario-repeater 5 by thehybrid coil H2 and associated balancing network N2, includes the noisediscriminating lter 25 in its input, the three-electrode vacuum tubedetector s 26 having its input connected to the output of filter 25,relay 27 connected to the output of detector 25, so as to be operativelyenergized by operation of the latter, and the gain increase relay 28adapted to be operatively energized by operation of relay 2l, relay 28operating to connect the positive pole oi the grounded battery 29 togain-regulating condenser 20 so as to increase its positive charge andthus raise the gain of vario-repeater 5. The gain increaser circuit 22is arranged to operate in response to an increase in the level of thevoice waves in the transmitting circuit TC in front of vario-repeater 5,above a certain minimum value to increase the gain of vario-repeaterproportionately.

The gain increase disabler circuit 23, the input of which is connectedacross the transmitting circuit TC at the same point as the input of thegain decreaser circuit 2l, includes a three-electrode vacuum tubedetector 3Q and the gain increase disabler relay 3l operativelyresponsive to operation oi detector 33 to disable the gain increasercircuit 22 by opening the energizing circuit for the gain increase relay28. The gain increase disabler circuit 23 is arranged to operate inresponse to the waves in the ,transmitting circuit TC beyond the outputof vario-repeater 5 when their level reaches the desired value so as toprevent further increase of the gain of vario-repeater 5 under controlof the gain increaser circuit 22.

In addition, the Vogad includes circuits for preventing change in thegain of the vario-repeater 5 in the transmitting circuit TC by theassociated gain control circuit when the receiving Vodas circuit RV hasbeen operated in response to received signals or noise in the receivingcircuit RC. These circuits include a relay 54 in circuit with the relayi@ operating simul. taneously therewith in response to operation of thereceiving Vodas master relay i8, and the relays dfi and it arranged tobe supplied with' energizing current from batteries d6 and 41,respectively, when relay 5 is operated. Relay 44 is designed to bequick-operating and slow-releasing so that it will be operated quicklyin response to operation of the receiving Vodas master relay ldeither byspeech signals or noise Waves, and will remain operated during the usualpauses in speech transmission. hand, is designed to be slow-operatingbut quickreleasing, so that it will not respond to operation of thereceiving Vodas master relay I8 by momentary speech or noise impulsesbut will respond to operation of relay i8 in response to continuouslyreceived speech waves.

Relay lill, when operated, causes relay 48 to bel 2S of gain increaser22 preventing its operation to increase the gain of vario-repeater 5while the receiving Vodas circuit RV is locked up regardless of whetherthis is caused by received speech signals or noise.

Relay 45, when operated, causes a relay 50 to be operatively energizedby current from battery 5l to short-circuit the input of the ygaindecreaser circuit 25, disabling that circuit and removing battery Elfrom the grid of the gain increase disabler tube 3l? thereby holding thegain increase disabler circuit 23 operated and effectively preventingany increase in the Vogad gain during the period in which the receivingVodas circuit RV is operated by received speech Waves. y

The volume control device 9 in the receiving circuit RC may be amanually adjusted arrangement, or is preferably an automatic adjustingarrangement such as disclosed in British Patent Relay 45, on the other'No. 381,831, Which functions at all times byinserting suicient loss tomaintain a safe margin wherein received energy in circuit RC which is soweak as to fail.y to operate control RV will also, at least, just failto cause false operation of control TV when parts of this energy reachcircuit TC' as echo energy. Y

The transmission devices and circuits in the radio telephone controlterminal as shown in the drawing, as described above, are such as arecommonly used in radio control terminals. The remaining portions of thecircuit illustrated in the drawing constitute one embodiment of thenoise protection circuit of the invention. As illustrated, this noiseprotection circuit includes three delayed operating relay controlcircuits A, B and C, shown within the dot-dash box labeled Noiseprotection control, controlled respectively from the transmitting Vodasmaster relay circuit including relay i3, the transmitting Vodas relay 3lwhich operates in a manner closely related to the operations of theswitching relay circuit including relays i4 and l5, and in combinationfrom relay 3l and the gain increaser circuit 22 of the Vogad in thetransmitting circuit TC, each of which delayed operating relay circuitscontrols the operation of the quickoperate quick-release action relay32, which, in turn, controls the transmitting Vogad gain.

The relay circuit A includes a master relay 33 in circuit with themaster relay i3 of the transmitting Vodas circuit TV and operatingsimultaneously therewith, and a relay 34 adapted to be operativelyenergized from battery 35 When relay 33 is operated. The operation ofrelay 3d causes operating current to be supplied to the winding of theaction relay 32 from battery 36. By suitable design of relay Sli to makeit slow to operate in the required' degree, the relay control circuit Ais made to cause operation of the action relay 32 in response tocontinuous operation of master relay 33, and therefore of the Vodasmaster relay i3, by the output current of the transmittingamplifier-detector I2 for a time interval of about 2 seconds after thetransmitting Vodas master relay i3 operates.

The relay control circuit B includes relay 3l, which is in circuit withand a part of the transmitting Vodas switching system with relays llland l5 and operates simultaneously with the latter relays when operatingcurrent is supplied from battery 38 in response tooperation of themaster Vodas switching relay I3, and the relay 39 arranged to besupplied with operating current from the battery ill when relay 3loperates. Operation oi the relay 39 also causes energizing current to besupplied to the Winding of the action relay 32 from battery 36. Byproper design oi the relay 39 to make it slow-operating to the requireddegree, the control circuit B is made to cause operation of the actionrelay 32 in response to continuous operation of relay 3l for a timeinterval of about l0 seconds.

The relay control circuit C includes relay 3l' and relay 4l which issupplied with energizing current from battery 'l2 through the backcontacts of the gain increase relay El (when that relay is unoperated)and the front contacts of relay 3l when the latter relay operates undercontrol of the transmitting Vodas master relay E3. The operation oirelay 4l also causes energizing current to be supplied to the winding oiaction relay 32 from battery 3G. By proper design of the relay di tomake it slow-operating to the required degree, the control circuit C ismade to cause operation of the action relay 32 in response to continuousoperation of relay 3l for a time interval of about 2 seconds providedthe Vogad gain increaser relay 2 has not operated.

The relays 34, 39 and di are designed to be quick-releasing so that thequick-releasing action relay S2 will return quickly to the unoperatedcondition when the transmitting Vodas switching relays release.

The operation of the control circuit C will be prevented if the gainincreaser circuit 22 has been previously operated by applied voice wavesfrom the transmitting circuit TC, to open the back contacts of the gainincrease relay 2l', thereby breaking the connection of battery i2 to theWinding of relay lll.

Operation of the action relay 32 in response to operation of any one ofthe control circuits A, B or C will cause the relay contacts to beclosed to connect the discharge resistance i3 in shunt with thegain-regulating condenser 2li of the Vogad. Then, the condenser will bedischarged through that resistance to reduce the gain of thevario-repeater 5 in an amount dependent on the time during which theaction relay 32 remains operated, that is, until the amplitude level oithe waves in the output of the variorepeater 5 impressed on thetransmitting Vodas circuit TV is suflciently reduced to cause therelease of the master relays i3 .and 33. Preferably, the time constantof the circuit (discharge resistance' 43) controlled by the action relay$2 should be made such that the gain ci the Vogad is reduced severaldecibels more than is sumcient to release the Vodas relays in order toavoid any appreciable eiiects like hunting.

The manner in which the circuits of the invention as described aboveoperate to provide protecticn against the various types of noisediiculties occurring in the operation of a radio terminal may beexplained as follows:

Considering the room noise problem, it will be assumed, as is generallythe case, that the transmitting Vogad gain is at some moderately lowvalue as adjusted properly by the last speech currents transmitted. Withthe transmitter of the talking party left open during a lull inconversation, it will be assumed that a burst of room noise then occursin suiiicient magnitude to operate the Vogad gain increasermeinentarily, whereupon the Vogad gain may increase some amount. Fromthis, or from successive but generally infrequent bursts of room noise,the gain of the Vogad may become high enough to canse virtual lock-up ofthe transmitting Vodas relays E3, 3i, it and it on the lower backgroundof room noise. When that occurs and persists for as long as 2 seconds,the slow-operate (2 seconds) relay control circuit C will operate tocause operation of the action relay 32, provided the gain increase relay2l has not in the meantime been operated to disable that circuit.

The gain increaser in any Vogad should have a sensitivity so that itwill operate at least on the strong peaks of any speech for which theVogad is intended to adjust, and these peaks should ocur at least onceevery second during continuous now of speech current. It is prac ticallyunavoidable that the gain increaser will operate occasionally, butcertainly not so regularly on random bursts of noise. On speech then,the control relay circuit C is effectively disabled by the gain increaserelay il', whereas on the usual types or room noise this relay circuitis seldom operated again within 2 seconds so 'Vodas circuit as soon aspossible.

that the control C is permitted to operate the action relay 32 andreduce the Vogad gain as it should. Thus, the undesirable room noiseoperation can be eectively and automatically detected and compensatedfor, in most cases Within 2 seconds of its occurrence.

Certain other types of noise, however, may not be eective in operatingcontrol circuit C. It has been observed that in ordinary conversation,

'the speech ows are so broken or grouped that the Vodas relays 3l, ldand i5 are practically never operated continuously for as long as 10seconds. Thus, any type of noise, intermittent or steady, Which, in sofar as the Vodas relays 3l, id and E5 are concerned, tends to bridge thegaps between the speech flows in an outgoing continuous train of speech,will operate control circuit B and thus operate the action relay 32within atime interval of l seconds after the Vodas switching relays 3l,M and i5 become operated. In the absence of speech currents in thetransmitting circuit TC, the relay control circuit B would be eiiective,of course, on any intermittent noise whose gaps between the successiveoperating peaks did not exceed .15 second in a continuous lll-secondperiod, the time of .i5 second being the slow-release time of thetransmitting Vodas relay 37, required in connection with the particulartransatlantic radio telephone terminal to which the circuits of theinvention were applied.

t has been further observed that in normal operation, the transmittedspeech currents will not hold the transmitting Vodas master relay l3-operated solidly for a continuous period as long as 2 seconds. Thus, anysteady noise tending to hold the master relay i3 operated continuouslyfor 2 seconds would operate relay control circuit A and thus the actionrelay 32. This type of noise would also, of course, operate relaycontrol circuit B if it persisted for seconds, but when it is sodenitely and quickly detected it is desirable to make any necessarycorrection in the sensitivity oi the transmitting This type of noisewould also be detected in 2 seconds by control circuit C if the noise atthe input of the Vogad was low enough in magnitude so as to fail tooperate the gain increaser 22.

When any oi these three noise protector circuits A, B or C causesoperation of the action relay 32, a connection through its contactsconnects the discharge resistance Il?. across the gain-regulatingcondenser C, which operates to reduce the charge on the gain-controllingcondenser 2@ in the Vogad so as to make the necessary reduction in theVogad gain. When the gain is reduced sufficiently, the Vodas relays I3,3l, Hi and l5 will release, in turn causing release of the action relay32 and stopping further unnecessary reduction of the Vogad gain. Thetime action of this gain decreasing arrangement can be adjusted by thechoice of the value of the resistance l2 in the leakage path of the gaincontrol condenser 29, so that in relation to the slowuelease time cirelay 3l (and the action relay 32), the gain reduction will be desirablycarried several decibels beyond the just release point and thus providea necessary margin of safety. This time action can be arranged, ofcourse, so that following an action of the noise protection circuits,the Vogad is immediately available to make any desired gain change.

To obtain optimum protection against noise difculties, it has been foundexperimentally that important only because noises of the types neces-`sary to cause their operation occur rather infrequently.

It is to be understood that the values for the delayed operating timesof the control circuits A, B and C speciiied above are given by way ofexample only, and that these operating times may diler radically fromthe values given, depending cn the constants of the particular Vodascircuits with which these circuits are employed.

The action relay 32, of course, may be ar,- ranged to operate on avariable gain amplier or Vogad located in the transmitting Vodas circuitTV to produce the required change in sensitivity to cause its releas-efrom abnormal operation on the diiT-erent types oi noise, instead of onthe Vogad used in the transmitting circuit TC, in a manner similar tothat described for the latter device. Also, similar results can beobtained by causing the operation of the action relay to switch Xedlosses of the proper values into the transmitting circuit TC or thetransmitting Vogad circuit TV. This may be accomplished, for example, byproviding resistance pads of the proper values in these circuits, whichare normally short-circuited, and having the action relay 32,`whenoperated, remove the short circuits.

Other mcdications of the circuits illustrated and described which arewithin the spirit and scope cf theinvention will occur to personsskilled in the art. v

What is claimed is:

l. In combination with a circuit transmitting signal waves and subjectto interfering noise waves of different types, and an associatedwaveoperated switching device operating differently on said signal wavesand sai-d noise waves, means for minimizing false operation of saidsvi/'itching device by the applied noise waves, comprising a pluralityof control circuits unresponsive to operation oi said device by thesignal waves but respectively responsive to characteristic abnormaloperation of said device by applied noise waves of a diierent one ofsaid types, to adjust the sensitivity oi' said device to prevent furtherfalse operation oy the applied noise waves.

2. In combination with a wave-operated switching device supplied from asource of signal waves intermittent in character and subject tocomparatively steady interfering noise waves of different types, meansfor minimizing false operation of said device by said noise wavescomprising a plurality of control circuits respectively'responsive tocontinuous operation of said device for diierent abnormally long timeintervals, each characteristic of operation by one of said differenttypes of noise, and eachlonger lthan would occur in response to appliedsignal waves alone, and means responsive to operation of any one of saidcontrol circuits to adjust the sensitivity of said device to prevent itsfurther false operation by the applied noise waves.

3. In combination, a line transmitting voice signal waves and subject tocomparatively steady interfering noise waves of diierent typesas regardsduration and frequency of occurrence, a voice-operated switching deviceconnected to said line, a plurality of control circuits respectivelyoperatively responsive to continuous operation of said device for anabnormally long time interval longer than would occur in response tovoice signal waves alone, caused by `a different one of saiddifferenttypes of noise, and means vresponsive to operation of any one of saidcontrol circuits to reduce the sensitivity of said device sufficientlyto prevent further false operation'by the applied noise waves.

4. In combination, a circuit transmitting voice signal waves and subjectto comparatively steady noise waves of diierent types as regardsduration and frequency of occurrence, a Wave-operated switching deviceconnected to said circuit, and means to minimize false operation of saidswitching device by said noise waves comprising a plurality of controlcircuits having diiierent delays in operation, eachoperatively'responsive to continuous operation of said switching devicefor a dii-ferent interval of time longer than would occur in response toapplied voice signal waves alone, respectively characteristic ofoperation by noise waves of a diiierent one of said types, and

means responsive to operation of any one of said control circuits toreducev the sensitivity of said switching 4device sufficiently to causeits release. Y

5. In a two-way telephone system comprising oppositely directed one-waytransmission paths for the telephone signals transmitted in oppositedirections, which are subject to comparatively steady .interfering noisewaves of dierent types as regards duration and frequency of occurrence,and a voiceoperated switching circuit connected to one path, operatingto control transmission in said paths so as to prevent singing andsuppress echoes, means to minimize-false operation vof said device bysaid noise waves comprising a plurality of control circuits respectivelyoperatively responsive to continuous operation of portions of saidswitching circuit for diierent intervals of time longer than would occurin response to appliedtelephonic signals alone, e4 spectivelycharacteristic to operation by a different one of said types of noisewaves, and means responsive to operatic-n of any one of said controlcircuits to reduce the sensitivity of said device suiciently to causeits release.

6. The system of claim 5 in which said voiceoperated switching devicecomprises an ampliiier-detector connected to one oi said one-way paths,a master relay circuit controlled by operation of saidampliiier-detector and a switching relay circuit responsive tooperation'oi said master relay circuit to properly control transmissionin said paths, .and said control circuits comprise relay chains havingdifferent delays in operation respectively determined by said differenttypes of noise waves, the operations of which are respectivelycontrolled from said master relay circuit and said switching relaycircuit. Y

7. In a two-way telephone system comprising oppositely directed one-wayvpaths for the telephonie signals transmitted in opposite directions,which are subject to comparatively steady interiering noise waves ofdifferent types as regards duration and frequency of occurrence, avoiceoperated switching circuit consisting of an ampliiier-detectorconnected to one of said paths so as to be supplied with the telephonicsignals and noise waves therefrom, a master' relay circuit responsive tooperation of said amplifier-detector, and a switching relay circuitresponsive to operation of said master relay circuit to disable thecluding a gain increaser portion, in said one path for automaticallyregulating the sensitivity of said switching circuit in accordance withthe level of the telephonie signals` in said one path, and means forminimizing false operation of said switching circuit by the appliednoise waves comprising a plurality of control circuits having respectivedelays in operation determined by the characteristics of the diiilerenttypes of noise waves, one of these control circuits being con trolledfrom said master relay circuit and two others from said switching relaycircuit, and means controlled by operation of said gain increaser ofsaid Vogad for preventing operation of one of said two other controlcircuits.

8. The system of claim 7, in which the delay time of said controlcircuit controlled from said master relay circuit is of the order of 2seconds, the delay time of said one of said two other control circuitscontrolled from said switching relay circuit is of the order of 2seconds, and the delay time of the second of said two other controlcircuits is of the order of 10 seconds.

9. The system of claim '1, in which the delay in operation of said onecontrol circuit controlled from said master relay circuit is determinedby the duration of comparatively steady noise such as would hold saidmaster relay operated for an excessive period, the delay in operation ofone of said two other control circuits is determined by the duration ci'the usual room noise, and the -delay in operation of the other ofsaid'two other control circuits controlled from said switching relaycircuit is determined by the duration of intermittent noise whosefrequency of occurrence is such as to eiectively bridge the normalreleases of the relays in said switching relay circuit.

l0. In combination with a voice-operating system comprising a pluralityof wave-responsive devices which separately or in combination responddifferently as regards duration of continuous operating intervals to agiven train of voice waves, means to indicate continuous operation ofthe respective devices for predetermined time intervals longer thanpredetermined longest sustained operating intervals in response tonormal voice waves, respectively characteristic of operation bydifferent types of noise waves, and means responsive to said indicatingmeans to adjust said system to make it unobjectionably responsive tonoise waves.

LIONEL SCHOTT.

